The present invention relates to a liquid crystal display which has a good display quality and is low-cost.
Conventional liquid crystal displays will be explained below. Although CRTs had been the mainstream of displays, active matrix-type liquid crystal displays (hereinafter referred to as LCDs) have been becoming prevalent in these years. The LCD is a display which utilizes the light transmittance of a liquid crystal, and does not emit light by itself. The LCD produces a gray scale display by controlling the emission of a backlight provided on the back among a light-transmitting state, a light-blocking state, and an intermediate state between the two states.
The backlight on the back uses, as a light source, a cold cathode tube (a type of fluorescent lamp, hereinafter referred to as a CCFL) which is a line light source, or a light emitting element (hereinafter referred to as an LED) which is a point light source. The backlight spreads light emitted from the tube or element in a two-dimensional plane as uniform as possible and serves as a plane illuminant.
Available light source arrangements include a bottom emitting type which uses light sources while arranging them immediately below a plane, and an edge emitting type arrangement which arranges light sources at edges of a plane and guides light in a planar manner.
In the bottom emitting type arrangement, efficiency for light utilization is generally high, and a large number of light sources can be arranged. Accordingly, the quantity of emitted light can be increased. However, to suppress non-uniformity in light emission from the line light sources such as the CCFLs or the point light sources such as the LEDs and make the light emission uniform, it is necessary to arrange a light diffuser or the like at a certain distance from the light sources, and install a light scattering sheet or the like around the light sources to make the light emission uniform by light scattering. For this reason, the backlight needs to have a moderately large thickness.
On the other hand, in the edge emitting type arrangement, linear light sources are arranged at an edge section, and light is guided in a direction perpendicular to a light emitting plane and made uniform. This allows a reduction in the thickness of the backlight. However, since efficiency for light utilization is low, and the number of light sources which can be arranged is limited in order to reduce the thickness, the quantity of emitted light decreases.
In currently used LCDs, an LCD for a notebook computer or one as a monitor for a desktop computer mainly adopts the edge emitting type arrangement in terms of portability and setting. In contrast, an LCD used as a television receiver, i.e., so-called liquid crystal television adopts the bottom emitting type arrangement because a large quantity of light is required.
As described above, most of the existing backlights use the line light sources or point light sources as an illuminant. To use them as a maximally-uniform two-dimensional plane illuminant, light is scattered in space and made uniform. As a result, light beams emitted from a light emitting plane have a high degree of scattering, and directions in which the light beams are emitted are not the same.
It is desirable for a user of an LCD, however, that the brightness of the LCD be intense in the front where the user mainly uses the LCD and that light be condensed in a direction perpendicular to a plane as the emitting direction of light from the backlight.
For this reason, an optical sheet with a light condensing ability such as a prism sheet or an optical sheet having a lens structure as described in JP-A-10-241434 is often used above the light emitting side of a light scattering plate in the case of the bottom emitting type arrangement or on the light emitting side of a light guide plate which guides light in the case of the edge emitting type arrangement.
Examples of future improvement in the performance of a backlight include use of primary color LEDs which can enhance color reproductivity, improvement in a light emitting method for remedying motion artifacts in an LCD, and a reduction in cost.
LEDs are now mainly used as backlights of small and medium-sized LCDs. These LEDs are white LEDs using a combination of a blue LED or the like and a fluorescent substance. On the other hand, an LED backlight which uses the RGB primary colors is expected to be mainly used in a liquid crystal television and can enhance color reproductivity and represent deep red and green.
To use LEDs as the backlight of a liquid crystal television, as a large quantity of light is required, it is necessary to use a large number of primary color LEDs. In this case, the LED backlight inevitably has the bottom emitting type configuration. However, since there are wide variations in light emitting property among existing LEDs, and LEDs which are a point light source are used in the bottom emitting type configuration in which a distance for scattering and uniformization of light is short, non-uniformity in light emission is highly likely to occur.
In order to avoid this, in the backlight of a commercially available liquid crystal television which uses the primary color LEDs, the distance between the LED illuminant and a scattering plate is long, and one more scatting plate is installed between the LED illuminant and the scattering plate.
Non-uniformity in light emission also occurs if a light emission area of a backlight is divided into areas to allow the areas to individually emit light for the purpose of improving the moving picture quality of an LCD, and a partition-like structure is provided to limit light from the respective areas, as described in JP-A-2001-318614. Similarly, a reduction in the number of light sources such as a CCFL and LED for a lower cost causes non-uniformity.
Examples of means for eliminating non-uniformity in light emission in a backlight as described above without increasing the thickness of the backlight include a method for varying the transmittance of a scattering plate from one point to another as described in JP-A-2002-182208 and a method for laying two scattering plates on top of another as described in JP-A-2001-318614.
In the meantime, the prices for displays using LCDs, i.e., notebook computers, liquid crystal monitors, liquid crystal televisions and the like are now rapidly falling. Accordingly, a reduction in the cost of an LCD is an urgent necessity.
To fabricate a scattering plate with an in-plane transmittance distribution as described in JP-A-2002-182208, it is necessary to seamlessly join sections with different concentration distributions or solidify the scattering plate with different concentration distributions in the manufacturing process of the scattering plate. This requires highly advanced manufacturing process technology and increases the cost. Additionally, since the transmittance distribution differs depending on the product specification, it is necessary to produce scattering plates with different transmittance distributions for respective types of products. This also leads to a higher cost.
In JP-A-2001-318614, two scattering plates are used, unlike an ordinary backlight which uses only one scattering plate. The configuration of JP-A-2001-318614 is higher in cost than that of the ordinary backlight.
As described above, improvement in the performance of a backlight such as use of a primary color LED for enhancement of color reproductivity or adoption of a partition structure intended to improve moving picture quality leads to an increase in thickness for elimination of non-uniformity in light emission or an increase in the cost of the backlight. In a backlight having light sources, the number of which is reduced for a lower cost, if the cost for eliminating non-uniformity in light emission increases, the effect of the reduction is lost.